Environment Canada, Meteorological Service of Canada, Meteorological Research Branch

Development Division

CRTI Project # 02-0093RD

A. Lemonsu,S. Bélair, J. Mailhot, L. Tong

Numerical Set-upModel

versionRes. Grid Version Integration

Daytime IOP6Integration

Nighttime IOP9TimeSte

pGEM reg

operational15 km Reg. GEM320

PHY42July 16, 0000 UTC

48 hrsJuly 26, 0000

UTC36 hrs

450 s

GEM/LAM 2.5 km

201x201

GEM322PHY44

July 16, 0600 UTC42 hrs

July 26, 0600 UTC

30 hrs

60 s

GEM/LAM 1 km 201x201

GEM322PHY44

July 16, 1200 UTC36 hrs

July 26, 1200 UTC

24 hrs

30 s

GEM-LAM 250 m

OKC

GEM-LAM 1 kmGEM-LAM 2.5 km

15-kmGEM Regional

Numerical Set-up

Sensitivity tests:

Simulation “urban”, including the Town Energy Balance (TEB) model

and using a specific urban land-use land-cover classification

Simulation “no-urban”, replacing OKC by grassland (ISBA)

GEM-LAM 250 m

OKC

GEM-LAM 1 kmGEM-LAM 2.5 km

15-kmGEM Regional

Methodology of Evaluation

Regional-scale evaluation

Comparison between 2.5-km GEM/LAM outputs and operational observations in Oklahoma State:

▫Near-surface parameters: MESONET operational network

▫Upper-air profiles: operational soundings of Norman (South of OKC)

Urban-scale evaluation

Comparison between 1-km GEM/LAM outputs and Joint Urban 2003 database:

▫Urban micro-climate inside the streets: PWIDS, SPWIDS, PNNL networks

▫Urban boundary layer: Radars/sodars, and Soundings south and north of CBD

Regional-scale evaluationDaytime IOP6 Nighttime IOP9

Alt

itud

e (m

)

Regional-scale evaluation

Daytime IOP6

Alt

itud

e (m

)

Regional-scale evaluation

Nighttime IOP9

Nocturnal

Low Level Jet

Urban-scale evaluation Daytime IOP6

10 rural stations (MESONET)13 urban stations (PWIDS)

OKC

Tcanyon

TPWIDS

zbld

zbld

28 m

Urban-scale evaluation

Sensitivity of the urban model to the geometric parameters:

By increasing the building height, roads and walls absorb less solar radiation and air temperature inside the street-canyon decreases at daytime

zbld=60 m

zbld=60 m

Daytime IOP6

Not enough vertical resolution to capture the sharp inversion at the top of the well-mixed layer

Model soundings not clearly sensitive to the inclusion of TEB

Large impact of the meteorology

PNNL

ANL

CBD

Urban-scale evaluation Daytime IOP6

urban no urban

Potential temperature at z=50 m – 1600 LST

The daytime urban heat island is advected according to the mean flow

Urban effect on daytime IOP 6

urban no urbanA

B

50

500

1050

1600

2150

2700

3250

Alti

tud

e (

m A

GL

)

Urban effect on daytime IOP 6

The atmospheric boundary layer is slightly warmer, north-east of the city, in the urban simulation: development of an urban plume

A B A BCity

Potential temperature at z=50 m – 1600 LST

PNNL

ANL

CBD

Urban-scale evaluation Nighttime IOP9

Alt

itud

e (m

)

Quasi-neutral boundary layer at night above the city

ABL warmer downwind of the city

Too strong inversion near the surface in the model

urban no urban

Potential temperature at z=50 m – 0200 LST

Urban effect on nighttime IOP 9

urban no urban

Potential temperature at z=50 m – 0200 LST

Alti

tud

e (

m A

GL

)

Urban effect on nighttime IOP 9

The vertical structure of the atmospheric boundary layer is influenced by the presence of the city

50

500

A BCity A B

A

B

Conclusion

GEM, including TEB, simulates the urban microclimate of OKC’s downtown

At street level: Positive urban heat island at night

Negative urban heat island at daytime

Daytime IOP:

Good performance of the model

Weak impact of the cities on local dynamics

Local dynamics mostly driven by larger-scale dynamics and soil conditions around the city

Nighttime IOP:

Good performance of the model for near-surface temperature

Underestimation of the quasi-neutral boundary layer over the city

Larger effect of the city on the structure of the ABL

Atmospheric model

zatm

Vegetated canopy

ISBA

Air Temperature Diagnostic

In mixed environment (including vegetation + built-up covers), the near-surface air temperature is diagnosed using:

2-m air temperature above vegetation, diagnosed from Ta and Ts

(ISBA) air temperature inside the street-canyon (TEB)

Urban canopy

TEB